This invention generally relates to apparatus for sealing conduits and more particularly relates to a nozzle dam having a unitary plug for sealing the primary nozzles of a nuclear steam generator.
Before discussing the current state of the art, it is instructive first to briefly describe the structure and operation of a typical nuclear steam generator. In this regard, a typical nuclear steam generator generally comprises a vertically oriented shell and a plurality of inverted U-shaped heat transfer tubes disposed in the shell. Pressurized radioactive primary fluid (e.g., water), which is heated by the core of a nuclear reactor, flows through the tubes as nonradioactive secondary fluid (i.e., water) of lower temperature circulates around the tubes. The steam generator further comprises a lower plenum defining an inlet plenum chamber and an outlet plenum chamber. A first leg of each inverted U-shaped heat transfer tube is in fluid communication with the inlet plenum chamber and the second leg of each inverted U-shaped heat transfer tube is in fluid communication with the outlet plenum chamber. Attached to the lower plenum and in fluid communication with the inlet plenum chamber is an inlet primary nozzle for delivering the primary fluid into the inlet plenum chamber. Moreover, attached to the lower plenum and in fluid communication with the outlet plenum chamber is an outlet primary nozzle for allowing the primary fluid to exit the outlet plenum chamber and thus exit the steam generator in the manner disclosed hereinbelow. In addition, the steam generator also includes a plurality of relatively small diameter manway openings in the lower plenum for allowing access to the inlet and outlet plenum chambers so that maintenance can be performed in the steam generator. In this typical nuclear steam generator, each manway opening has a diameter substantially less than the inside diameters of the inlet and outlet primary nozzles.
During operation of the steam generator, the primary fluid, which is heated by the nuclear reactor core, flows from the nuclear reactor core, through the inlet primary nozzle and into the inlet plenum chamber. The primary fluid then flows into the first leg of each heat transfer tube, out the second leg of each heat transfer tube and then into the outlet plenum chamber, whereupon it exits the outlet primary nozzle. Moreover, as the primary fluid exits the outlet primary nozzle, it is returned to the nuclear reactor core to be reheated. It will be appreciated that as the primary fluid flows through the heat transfer tubes, heat is transferred from the primary fluid to the secondary fluid for producing steam in a manner well known in the art of nuclear-powered electricity production. Such a nuclear steam generator is more fully disclosed in U.S. Pat. No. 4,079,701 entitled "Steam Generator Sludge Removal System" issued Mar. 21, 1978 to Robert A. Hickman et al.
Periodically, it is necessary to shut down the nuclear reactor core for refueling. At that time, it is also advantageous to perform maintenance on the steam generator. Such maintenance, for example, may be to plug and/or sleeve potentially degraded steam generator tubes or to decontaminate the steam generator. During maintenance activities, the reactor is partially drained of primary fluid to a level that is below the elevation of the inlet and outlet primary nozzles of the steam generator. However, it should be appreciated that the nuclear reactor should never be drained to a level that uncovers the reactor core. This process of partially draining the reactor also drains the heat transfer tubes and the inlet and outlet plenum chambers. After the heat transfer tubes and the inlet and outlet plenum chambers are drained of primary fluid, nozzle dams are inserted through the relatively small diameter manways and installed in the inlet and outlet primary nozzles to block the nozzles. Once these dams are in place, the nuclear reactor and the refueling cavity can then be refilled with primary fluid for the refueling operation, without interfering with maintenance activities being performed in the steam generator lower plenum because the nozzle dams prevent radioactive primary fluid from rising into the lower plenum of the steam generator. Also, once the nozzle dams are installed, the steam generator can be subjected to a chemical decontamination procedure without contaminating the nuclear reactor with contaminants removed during the decontamination of the steam generator because the inlet and outlet primary nozzles are blocked by the nozzle dams. Moreover, it is cost advantageous to simultaneously perform steam generator maintenance while the reactor core is being refueled rather than performing these activities in seriatim. This is true because it is necessary to shut down the reactor core during refueling or during steam generator maintenance. Of course, revenue-producing electricity is not generated while the reactor core is shut down. Each day the reactor core is shut down results in approximately $200,000 in lost revenue. Therefore, simultaneously performing reactor refueling and steam generator maintenance activities reduces the time the reactor core is shut down, thereby recapturing a portion of the approximately $200,000 each day in revenue that would otherwise be lost.
As stated hereinabove, the manway openings are typically substantially smaller than the inside diameter of the primary nozzles. Therefore, it has been necessary in the art to use foldable nozzle dams that can be folded to fit through the relatively small diameter manways and then unfolded to be disposed in the primary nozzles. The elastomeric seals of such nozzle dams are subjected to stresses, such as bending stresses, as the nozzle dams are folded and unfolded. Repeated folding and unfolding of the nozzle dam may increase the risk that the seals will loose their sealing ability over time after being subjected to these stresses as the nozzle dam is repeatedly folded and unfolded.
Steam generator nozzle dams are known. One such nozzle dam is disclosed in U.S. Pat. No. 4,637,588 entitled "Non-Bolted Ringless Nozzle Dam" issued Jan. 20, 1987 in the name of John J. Wilhelm et al. and assigned to the assignee of the present invention. This patent discloses a nozzle dam having one or more seal assemblies, each including a foldable circular seal plate encircled with an inflatable seal which is disposable in frictional engagement with the nozzle wall. It is a significant aspect of the Wilhelm et al. device that the seal plate be foldable because the nozzle dam must fit through the steam generator manway which has a diameter substantially less than that of the nozzle into which the nozzle dam will fit.
Another nozzle dam is disclosed in U.S. Pat. No. 4,671,326 entitled "Dual Seal Nozzle Dam and Alignment Means Therefor" issued Jun. 9, 1987 in the name of John J. Wilhelm et al. and assigned to the assignee of the present invention. This patent discloses a seal assembly including a foldable circular seal plate having a center section hingedly connected to two side sections. The seal plate is foldable for inserting the nozzle dam through the relatively small diameter steam generator manway. This patent also provides that a worker is present in the steam generator to unfold and insert the nozzle dam into the primary nozzle.
Therefore, a problem in the art has been to provide a nozzle dam that need not be folded and unfolded, so that the sealing ability of the seals attached to the nozzle dam are not compromised.
Another problem in the art has been to provide a nozzle dam having a unitary plug that is remotely installable so that maintenance personnel need not be present in the radioactive environment of the steam generator to manually insert the nozzle dam in the primary nozzles.
Thus, although the above recited patents may disclose nozzle dam devices, these patents do not appear to disclose a nozzle dam having a unitary plug capable of being remotely inserted through a relatively small diameter steam generator manway without the necessity of the nozzle dam and plug being foldable and unfoldable.
Therefore, what is needed is a nozzle dam having a unitary plug for sealing the primary nozzles of a nuclear steam generator, the unitary plug capable of being remotely inserted through a relatively small diameter steam generator manway without the necessity of the nozzle dam and plug being foldable and unfoldable in order to insert it through the manway opening.